In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an imagewise way on a image receiving substrate, the latter not bearing any imagewise latent electrostatic image. The substrate can be an intermediate endless flexible belt (e.g. aluminium, polyimide, etc.). In that case the imagewise deposited toner must be transferred onto another final substrate. Preferentially the toner is deposited directly on the final image receiving substrate, thus offering a possibility to create directly the image on the final image receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed by Pressman in U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer; PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and PA1 at least one row of apertures. PA1 the need for a rather high voltage on the control electrode to close the apertures surrounded by said control electrodes (i.e. to overcome the applied propulsion field), PA1 expensive electronics for changing the overall density between maximum and minimum density, making the apparatus complex and expensive, PA1 easy contamination or even clogging of the printing apertures by toner particles. PA1 1. isolated wires in a cross direction; PA1 2. a flexible PCB with only control electrodes in the cross direction and PA1 3. a flexible PCB with common shield electrode and control electrodes in the cross direction. The different systems according to this disclosure make it possible to change the propulsion field in a group of apertures, tuning the density by setting the voltage of the different control electrodes, and require only moderate printing voltages.
Each control electrode is formed around one aperture and is isolated from each other control electrode. Hereinafter a printhead structure as describe immediately above will be referred to as "classical" printhead.
Selected potentials are applied to each of the control electrodes, while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a support for an image receiving substrate projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a image receiving substrate, interposed in the modulated particle stream. The image receiving substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the image receiving substrate. A DC field is applied between the printhead structure and a single back electrode on the support for the image receiving substrate. This propulsion field is responsible for the attraction of toner to the image receiving substrate that is placed between the printhead structure and the back electrode.
Printing with an engine as described in U.S. Pat. No. 3,689,935 is quite well possible, but shows also some drawbacks. Important drawbacks, that have been addressed in several disclosure are:
The drawbacks, mentioned above, result in a poor output quality, especially eveness of the density in solid density areas, and a bad long-time stability if the printing engine is used over several hours.
To overcome these problems several modifications have been proposed in the literature.
In U.S. Pat. No. 4,912,489 the conventional positional order of shield electrode and the control electrode--as described by Pressman--has been reversed (i.e. the shield electrode faces the image image receiving substrate and the control electrodes the toner source). This results in lower voltages needed for tuning the printing density. In a preferred embodiment, this patent discloses a new printhead structure in which the toner particles from the toner delivery means first enter the printhead structure via larger apertures, surrounded by so-called screening electrodes, further pass via smaller apertures, surrounded by control electrodes and leave the structure via a shield electrode.
In EP-A-0 587 366 an apparatus is described in which the distance between printhead structure and toner delivery means is made very small by using a scratching contact. As a result, the voltage needed on the control electrodes to close the apertures surrounded by said control electrodes (i.e. to overcome the applied propulsion field) is very small. The scratching contact, however, demands a very abrasion resistant top layer on the printhead structure.
An apparatus working at very close distance between the printhead structure and the toner delivery means is also described in U.S. Pat. No. 5,281,982. Here a fixed but very small gap is created in a rigid configuration, making it possible to use a rather low voltage to select wanted packets of toner particles. However, the rigid configuration requires special electrodes in the printhead structure and circuits to provide toner migration via traveling waves.
In U.S. Pat. No. 4,568,955 e.g. a segmented support for an image receiving substrate, comprising different galvanically isolated styli as control back electrodes is used in combination with toner particles that are migrated with travelling electrostatic waves. The printing can proceed with lower voltage, but resolution is limited and the image quality depends quite strongly on both the environmental conditions and properties of the image receiving substrate.
In U.S. Pat. No. 4,733,256 some of the problems cited above are addressed by the combination of a "classical" printhead structure, i.e. a printhead structure as described in U.S. Pat. No. 3,689,935, and a segmented back electrode (control back electrode), comprising different isolated wires and carrying the image receiving member. For a line printer the density can be tuned by selecting an appropriate voltage for shield electrode, control electrode and control back electrode wire.
In U.S. Pat. No. 5,036,341 a device is described comprising a screen- or lattice shaped control back electrode matrix as segmented support for an image receiving substrate. This apparatus has the advantage that matrix-wide image information can be written to the image receiving substrate, but it also suffers from the environmental influences and those caused by the nature of the image receiving substrate.
To overcome these drawbacks in U.S. Pat. No. 5,121,144 another device wherein the segmented back electrode without printhead structure was changed into a two part electrode system, having a printhead structure electrode and a back electrode structure. A first part was placed between the toner delivery means and the image receiving substrate and consisted of parallel, isolated wires, being used as printhead structure. A second part consisted of another set of parallel wires, arranged orthogonally with respect to the first wires and was used as back electrode structure. The support for the image receiving substrate or back electrode structure in all examples consists of isolated wires which are oriented in one direction. As printhead structure, there are described three different configurations:
In U.S. Pat. No. 5,402,158 the above indicated printhead structure 2 (namely a flexible PCB with individually controllable control electrodes without shield electrode) is also used in combination with a non-segmented ("classical") back electrode. This printhead structure, however, has the disadvantage that frictional charging can occur leading to image instabilities for long-term printouts.
This last disadvantage has been partially overcome, as described in U.S. Pat. No. 5,307,092, by the application of a grounded antistatic overcoat over said single-plane-electrode printhead structure, i.e. in stead of using a metal conductor as shield electrode, an antistatic layer is used.
According to U.S. Pat. No. 4,491,855 the image density can be enhanced by the introduction of an AC-voltage, applied to the toner conveying member. As a result, shorter writing times are possible. But, to obtain a reduced image density, the quite elevated voltage levels must be applied.
In U.S. Pat. No. 5,229,794 and the EP-Application 710 897 a printhead structure with individually controllable control electrodes and shield electrodes around every aperture is described. These printhead structures have the advantage that accurate control over potential values at the outer surfaces of the printhead structure are possible, but the electronics needed to drive such a complex printhead structure make a device making use of said printhead structure quite complex and expensive.
The disclosures mentioned above, do solve some of the problems present in the original DEP (Direct Electrostatic Printing) device, but in general the combination of low voltage addressable printing apertures combined with low contamination just fulfil one or a few of the different requirements for an inexpensive DEP device, delivering high-quality images with stable densities.
There is thus still a need to have a DEP system, based on a simple apparatus, yielding high quality images in a reproducible and constant way.